It's not much important the song in it, the important thing is to use always the sames. Do this:
choose a song (doesn't really matter) and don't listen it too often, then when you have been in the field for a time and you have a confidence state listen your song with your mp3. Repeat it for a lot of times. When you have done it you can start using it before approaching. It will be super effective.
It's basic NLP anchoring. If every time that you are feeling confident you hear this song your brain thinks "oh man, every time that I am feeling confident I hear this song, maybe it have something to do with it", then the song and the feeling are related in your brain. you feel confident >> you hear the song ; you hear the song >> you feel confident.
It's like when you used to listen a song when you were sad, and now listening to it makes you feel sad.
Hope that helped.
